An Emergency Notification System allows emergency services to reach out to residents. Typically examples are in case of evacuations due to wild-fire, or noticed to stay inside due to hazardous material incident, and so on. In the days of land lines (where phone numbers were tied to a residence) things were simpler and emergency services could know phone number/resident information from telecommunications providers, but that is no longer the case.
Today with mobile phones and Internet telephony there is no easy way to know all the phone numbers in an area. Further, electronic communications (such as e-mail or TXT) are valuable for some people, especially when they are away from home (e.g. at work.) These new communications approaches are valuable but the resident now needs to register with their local emergency services organizations. This is typically done by county.